Theory of Anomalous Electron Spin Resonance Spectra of Free Radicals in Solution. Role of Diffusion-Controlled Separation and Reencounter of Radical Pairs

TLDR

The study proposes that anomalous spin‑state populations in transient radical ESR spectra arise from a sequence where radicals collide or are formed together, diffuse apart, and then undergo a second nonreactive collision. The mechanism involves singlet–triplet mixing during the interval between encounters and singlet–triplet splitting at the second encounter, with the relative dominance of hyperfine or Zeeman interactions determining whether mixed or purely emissive/absorptive spectra result, and the extended time between separation and return allowing weak magnetic interactions to alter the radical pair wavefunction. The model predicts that this encounter sequence increases the likelihood of radicals ending in spin states correlated with the initial pair, explains why longer separation times enhance this effect compared to solvent‑cage trapping, shows that uncorrelated radical collisions yield polarizations identical to an antibonding triplet pair, and matches experimental ESR spectra in multiple cases.

Abstract

It is proposed that the anomalous spin state populations observed in electron spin resonance spectra of transient radicals in liquids are produced by the following sequence of encounters between two radicals: The radicals either collide or are produced together by a chemical reaction and, instead of recombining, diffuse apart and subsequently undergo a second nonreactive collision. During the interval between the first and second encounters the radical pair wavefunction changes under the influence of singlet–triplet mixing by the magnetic interactions within the radicals. At the second encounter the singlet–triplet splitting associated with the valence bonding–antibonding interactions between the radicals produces further changes in the radical pair wavefunction. The net effect of these changes in the radical pair wavefunction is to increase somewhat the probability of finding the finally separated radicals in those spin states which correlate separationwise with the initial state of the radical pair. The advantage of this mechanism over a somewhat similar mechanism in which singlet–triplet mixing and singlet–triplet splitting operate simultaneously on a radical pair trapped briefly in a solvent cage is that the relatively long time between separation and return of the diffusing radicals gives the weak separation-independent magnetic interactions ample time to change the radical pair wave function. If the nuclear hyperfine interactions dominate the magnetic interaction mixed emission–absorption spectra result; purely emissive or absorptive spectra result if the electron Zeeman interactions predominate. Collision of independently produced radicals with uncorrelated spins gives the same polarizations as a radical pair produced in an antibonding triplet state. Good agreement between theory and experiment has been obtained for several cases.